Modeling of the mechanical behavior of polytetrafluoroethylene (PTFE) compounds during their compaction at room temperature

PTFE is not melt-processible. One of the production methods of PTFE parts consists in the powder compaction at room temperature followed by a thermal treatment, the sintering. Fillers can be added to the virgin powder. In order to be able to predict the properties of the obtained green parts and to have a reliable tool to optimize the parameters of the process, modeling of the mechanical behavior of PTFE compounds during their industrial pressing in big billets is proposed. Experimental characterization is made thanks to a 3D compaction tool, installed in the triaxial machine ASTREE. From original and complex loadings, a Drucker-Prager/cap model is identified, where the variations of density and properties are described. In addition to the elastoplastic model, a phase transformation in the crystalline structure at ambient temperature under pressure is experimentally characterized, modeled and implemented in the FE code. Then the interaction of the PTFE with a metallic counterpart is described as a friction between the PTFE transfer film and the bulk PTFE, characterized by the internal friction coefficient of the Drucker-Prager line once the cohesion of the material is reached. No additional parameter needs to be identified. Two laboratory tests allow the validation of the model, an instrumented œdometric tool and an original ‘V’ tool where the displacement field of the material during the compaction is measured by Digital Image Correlation. FE simulation of the whole compaction process is finally made and compared to industrial data. First links between the compaction and the sintering are established by a characterization of the crystalline texture thanks to XRD.